The ability to increase the storage capacity in magnetic recording is an ongoing concern. As the amount of information to be stored continues to increase, demands for higher density recording also continue to increase. Heat assisted magnetic recording (HAMR) is one proposed technology for increasing the storage density of conventional magnetic recording devices. Heat assisted magnetic recording combines facets of both optical and magnetic recording in an effort to increase storage capacity.
Conventional hard disc drives rely on a magnetic field produced by a small recording pole formed on a recording head. The recording head and recording pole are on a slider that “flies” across the surface of the disc as the disc spins. The magnetic field from the small recording pole needs to be sufficient to overcome the coercivity of the magnetic recording medium in the disc in order to define the recorded bits along the recording track in the medium.
As the storage density of disc drives increases, the size of the recorded magnetic marks in the recording medium must correspondingly decrease. As used herein, a mark is simply a recorded feature and, depending on the encoding scheme, may be of varying lengths, e.g., 1, 2, 3, . . . bits. Additionally, the individual magnetic grains which make up a recorded mark must also decrease in size to maintain approximately the same number of grains per bit cell to assure a sufficient signal-to-noise ratio (SNR). However, as the volume of the magnetic grains decreases, the thermal stability of the grains will also decrease unless the coercivity of the recording medium is increased. The disc drive industry is rapidly approaching storage densities where the magnetic fields that can be generated by conventional recording poles will be insufficient to magnetically switch the magnetic grains in recording media with a coercivity large enough to ensure the thermal stability of recorded data for at least 10 years, which is an industry standard.
As previously noted, heat assisted magnetic recording is one proposed technique for circumventing this difficulty. Heat assisted magnetic recording reduces the coervicity of the magnetic grains only during recording by optically heating the spot to be recorded. Experiments with heat assisted magnetic recording have demonstrated that the optimum recording situation occurs when the optical spot is coincident with the magnetic recording field from the recording pole. Such coincidence of the optical spot and the magnetic field is possible when the substrate is transparent and the optical spot is approximately 1 micron by locating the optical head on the side of the recording medium opposite that of the magnetic recording head. However, for optical spots which are sub-wavelength, which are necessary for high storage densities, a near field light source must be used which requires the optical head to be located on the same side of the recording medium as the magnetic recording head. This raises another difficulty in that conventional magnetic recording poles are made up of materials having high permeabilities, such as FeCo alloys and the like, which are metallic and thus opaque to the light that is utilized to create the hot optical spot in the recording medium. Conventional recording poles thus do not permit co-location of the optical spot and the magnetic recording field on the recording medium.
One alternative which has been suggested is to generate the magnetic field by an electrical current rather than using a magnetic recording pole. For example, a copper coil may be lithographically deposited onto the bottom surface of a solid immersion lens or waveguide. The copper coil would not interfere with the light propagating through the center of the waveguide, and would still be capable of generating a magnetic field at the same location on the recording medium that the light from the waveguide is heating. Unfortunately, the magnetic fields that are reasonably generated by a coil are on the order of hundreds of Oersteds, which is approximately ten times smaller than the magnetic fields capable of being generating by placing a permeable material within the electrical coil. Such small magnetic fields may not be suitable for a HAMR storage device because they would require that the optical spot heat the recording medium to very close to its Curie point to substantially reduce the coercivity of the medium and allow the grains to be magnetically switched. Furthermore, the fields applied by the magnetic recording head need to be significantly larger than the fields produced by the previously written information on neighboring tracks to avoid an undesired modulation or transition shift in the written data pattern. The ferromagnetic nature of the recording media, the desire to maintain a strong read-back signal, and adequate thermal stability leads one to use recording media with a large remenant magnetization, which for high track density recording results in fields up to a few hundred Oe.
The present invention is directed toward overcoming one or more of the above-mentioned problems.